Our research addresses structural, magnetic, and transport properties of novel materials with
an eye toward practical applications. This is done by calculating the properties of real materials
using first-principles computational techniques and testing the results by neutron scattering
and other measurements. Systems of particular interest include molecular solids such as the
fullerenes and cubane, nanoporous materials such as metal-organic frameworks for hydrogen storage,
frustrated magnetic systems such as the Kagome lattice and cuprates, novel superconductors
such as iron-pnictides, doped fullerenes and magnesium diboride, and nanomaterials such as
nanotubes and molecular magnets.

RECENT RESEARCH HIGHLIGHTS:
Phonon-driven superconductivity in
the vicinity of ferroelectric and charge density wave ordering in
La(O,F)BiS2
Phys. Rev. B 87, 020506(R) (2013)
Very recently a new family of layered materials containing BiS2 planes was
discovered to be superconducting at temperatures up to 10 K. These new systems
REOxF1-xBiS2 (RE=La, Nd, Pr, and Ce)
are structurally similar to the layered,
iron-based superconductors LaOxF1-xFeAs, and in both cases the
superconductivity is achieved by F-doping. In addition, band structure
calculations indicate the presence of strong Fermi surface nesting at
the wave vector (pi,pi), which is the hallmark property of the Fe-pnictides.
These similarities have raised the exciting question of whether or not
the superconducting mechanism in the BiS2 system is related to that
in the iron and have therefore generated enormous interest.
The fundamental question is whether or not the observed Tc in this
new system can be understood within a conventional electron-phonon
coupling framework, or is a more exotic mechanism responsible for
the superconducting pairing? We performed state-of-the-art first
principles calculations that directly address this question and
reveal several surprising findings. We show that the parent
compound LaOBiS2 possesses highly anharmonic, ferroelectric,
soft phonons at the zone center and a spontaneous polarization
of ~10 &mu C/cm2. Upon electron doping new instabilities appear
at (&pi/2,&pi/2) and (&pi,&pi), which causes the Bi/S atoms to order
into a one-dimensional charge density wave (CDW).
We find that BiS2 is a strong electron-phonon coupled superconductor
in close proximity to competing ferroelectric and CDW phases.
Our results suggest new directions with which to tune the balance between
these phases and increase Tc in this new class of materials.
Read more:
Ferroelectric soft phonons, charge density wave instability, and
strong electron-phonon coupling in BiS2
layered superconductors: A first-principles study, Taner Yildirim,
Phys. Rev. B 87, 020506(R) (2013).
Exceptional CO2 Capture in a Hierarchically Porous Carbon with
Simultaneous High Surface Area and Pore Volume
Energy Environ. Sci. 2013 ; DOI: 10.1039/C3EE42918K
A new group of highly hierarchical porous carbons (HPC) with high
surface areas up to 2734 m2/g and high total pore volumes up to
5.53 cm3/g have been synthesised by tailored carbonization of
various MOFs. The HPCs are highly sp2-bonded graphenic in
nature with a high portion of defective carbon structures. In most
cases, the CO2 uptakes in HPCs are higher than in their MOF
counterparts. The high pressure CO2 uptake of over 27 mmol/g
at 30 bar and 300 K in HPCs is one of the largest reported in the
literature for porous carbons. There seems a direct relationship
between the CO2 adsorption capacity and the surface area:
a CO2 uptake of 10 mmol/g for every 1000 m2/g
increase of surface area under 30 bar and 300 K.
Due to thermal/chemical stability and enhanced CO2
capture performance the HPCs are preferred
over their counterpart MOFs for PSA/VSA applications.
Our study also suggests that
MOF derived hierarchical porous carbons may be favourably
considered for other energy related applications, such as lithium-
ion adsorption-desorption in battery and supercapacitor electrodes.
Read more:
G. Srinivas, V. Krungleviciute, Z. Guo, and T. Yildirim,
Energy Environ. Sci. 2013 ; DOI: 10.1039/C3EE42918K
Methane Storage in Metal-Organic Frameworks: Current Records,
Surprise Findings, and Challenges
J. Am. Chem. Soc. 135, 11887 (2013)
We have examined methane storage in MOFs by performing a
comparative study of six of the most promising MOFs. To our surprise,
HKUST-1, an inexpensive MOF that is commercially available in gram scale,
exhibits record high total uptake of around 230 cc(STP)/cc at 35 bar and
270 cc(STP)/cc at 65 bar, meeting the U. S. Dept. of Energy?s new
volumetric target. We emphasize that MOFs with high surface
areas and pore volumes perform better overall. NU-111, for
example, reaches ~75% of both the gravimetric and volumetric targets.
We estimate that a MOF with surface area 7,500 m2/g and pore volume
3.2 cc/g could reach the current DOE gravimetric target of 0.5 g/g
while simultaneously exhibiting around ~200 cc/cc volumetric uptake.
One of the important challenges going forward is to find ways to
pack MOFs efficiently without collapsing pore volumes or to
synthesize MOFs that can withstand substantial mechanical pressure.
Read more:
Methane Storage in Metal-Organic Frameworks: Current Records,
Surprise Findings, and Challenges,
J. Am. Chem. Soc. 135, 11887 (2013)
Unusual and Highly Tunable Missing-Linker Defects in
Zirconium Metal-Organic Framework UiO-66 and
Its Effects on Gas Adsorption
J. Am. Chem. Soc. 135, 10525 (2013)
UiO-66 is a highly important prototypical zirconium metal?organic framework
(MOF) compound because of its excellent stabilities not typically found
in common MOFs. In this study, we show that by changing the concentration of
acetic acid modulator and the synthesis time, the linker vacancies can be
tuned systematically. We obtained samples with pore volumes ranging from
0.44 cc/g to 1.0 cc/g and BET surface areas from 1000 m2/g to 1600 m2/g.
These numbers for pore volume and surface area are ~150% and 60% higher
than the theoretical values of ideal crystal, respectively.
The linker vacancies have also profound effects on the gas adsorption
behaviors of UiO-66, in particular CO2. Finally, comparing the gas
adsorption of hydroxylated and dehydroxylated UiO-66, we found that
the former performs systematically better than the latter
(particularly for CO2) suggesting the beneficial effect of the
OH groups. This finding is of great importance because hydroxylated
UiO-66 is the practically more relevant, non-air sensitive form of this MOF.
Read more:
J. Am. Chem. Soc. 135, 10525 (2013)
Gram-scale, high-yield synthesis of a robust metal-organic
framework for storing methane and other gases
Energy Environ. Sci. 6, 1158 (2013)
We have synthesized and characterized a new metal-organic framework (MOF) material,
NU-125, that, in the single-crystal limit, achieves a methane storage density
at 58 bar (840 psi) and 298 K corresponding to 86% of that obtained with
compressed natural gas tanks (CNG) used in vehicles today, when the
latter are pressurized to 248 bar (3600 psi). More importantly,
the deliverable capacity (58 bar to 5.8 bar) for NU-125 is 67% of
the deliverable capacity of a CNG tank that starts at 248 bar.
This material was synthesized in high yield on a gram-scale in a
single-batch synthesis. Methane adsorption isotherms were measured
over a wide pressure range (0.1-58 bar) and repeated over twelve
cycles on the same sample, which showed no detectable degradation.
Adsorption of CO2 and H2 over a broad range of
pressures and temperatures are also reported and agree with
our computational findings.
See the media coverage:
Easing the Pressure on Gas Storage
Read more:
Energy Environ. Sci. 6, 1158 (2013)
Simultaneously high gravimetric and volumetric methane uptake
characteristics of the metal-organic framework NU-111
Chem. Commun. 49, 2992 (2013)
We show that the MOF NU-111 exhibits equally high volumetric
and gravimetric methane uptake, both within ~75% of the DOE targets at 300 K.
Reducing temperature to 270 K, the uptakes increase to 0.5 g/g and 284 cc(STP)/cc at 65 bar,
reaching DOE's targets. Adsorption of CO2 and H2 is also reported.
Simulated isotherms are in excellent agreement with experiment.
Read more:
Chem. Commun. 49, 2992 (2013)

Carbon capture is a critical component of the mitigation of CO2 emissions from industrial plants.
Investigations of the application of MOFs to adsorptive carbon capture have focused on their
appreciable storage capacities but fail to address the more pertinent issue of how MOFs
perform under common industrial separation processes that are at the heart of carbon capture.
Typical processes rely on swing adsorption and are limited to relatively low CO2
partial pressures such that the total pore volume and the surface area are under-utilized.
Here, we investigate the performance of a number of MOFs with particular
focus on their behavior at the low pressures commonly used in swing adsorption.
This comparison clearly shows that it is the process that determines which
MOF is optimal rather than there being one best MOF, though MOFs that possess
enhanced binding at open metal sites generally perform better than those
with high surface area. This work will be an important guideline for deciding
the best pair of carbon capture process and MOF material for optimum carbon capture.
Read more:
Carbon capture in metal-organic frameworks-a comparative study,
J. M. Simmons, H. Wu, W. Zhou and Taner Yildirim*,
Energy Environ. Sci., 2011, 4, 2177-2185
DOI: 10.1039/C0EE00700E
Fast and Clean Hydrogen Generation from AB-loaded MOF
Chem. Eur. J. , 20 April 2011, DOI: 10.1002/chem.201100090

Among many hydrogen storage candidate materials, ammonia borane (AB) has been considered
as one of the most promising ones because of its remarkably high hydrogen content
(19.6 wt.%), moderate decomposition temperature and stability. However, the direct
use of pristine AB in practical applications so far is prevented due to its slow
H2 release kinetics below 100 oC and the detrimental volatile byproducts;
ammonia, borazine and diborane.
In this communication, we report that AB intercalated metal-organic frameworks (MOFs)
not only enhance the hydrogen release kinetics but also suppress the
detrimental byproducts which is the most important outstanding issue
for practical fuelcell applications. The observed hydrogen release
kinetics and prevention of byproducts are found to critically
depend on the nature of metal-type and its coordination,
the size of the framework pore structure and the level of AB loading.
The results reported here bring us one step closer to using AB as a
hyrogen storage medium for the polymer electrolyte membrane fuel-cell applications.
Read more:
Nanoconfinement and Catalytic Dehydrogenation of Ammmonia Borane by
Magnesium-Metal-Organic-Framework-74, S. Gadipelli1, J. Ford,
W. Zhou, H. Wu, T. J. Udovic, and Taner Yildirim*,
Chem. Eur. J. 2011, April 20, DOI: 10.1002/chem.201100090
Graphene Oxide Framework Materials Suggested for Hydrogen Storage and Carbon Capture
Angew. Chem. Int. Ed. 2010, 49, 8902-8904

In this communication, we show that one-and-a-half-century-old graphene oxide (GO)
can be easily turned into a potentially useful gas storage material.
GO is a sheet of carbon atoms with many hydroxyl, epoxide and carboxyl surface groups.
In principle, hydrogen can be stored between layers of this lightweight material.
However, the challenge is to separate the layers without filling the space between them.
Here we show that by using the well-known chemistry between diboronic acids and hydroxyl groups,
GO layers can be linked together to form a new layered structure.
Such GOF structures have tunable pore widths, volumes, and binding sites depending
on the linkers chosen, and could exhibit interesting gas sorption properties.
As discussed in the manuscript, ideal GOF structures can adsorb hydrogen up to
6 wt% at 77 K and 1 bar, a value higher than any other porous material known.
Our synthesized GOF materials exhibit 9 kJ/mol and 32 kJ/mol isosteric heat of
adsorption for H2 and CO2, significantly larger than those found in
similar nanoporous materials. The nitrogen BET surface area reaches
a maximum at 470 m2/g. Despite this low surface area,
GOF exhibits 1 wt% H2 uptake at 1 bar. This is much less than what the 'ideal' GOF
structure can hold, suggesting that our initial synthesized
GOF materials could be significantly optimized in the near future.
Read more:
Graphene Oxide Framework Materials: Theoretical Predictions and Experimental Results,
J. W. Burress, S. Gadipelli1, J. Ford, J. M. Simmons, W. Zhou, T. Yildirim*,
Angew. Chem. Int. Ed. 2010, 49, 8902-8904
The Unprecedented Giant Magneto-Elastic Coupling in New Iron-based superconductors
T. Yildirim, Phys. Rev. Lett., 102, 037003 (2009)

From first principles calculations we unravel surprisingly strong interactions between arsenic ions
in iron-pnictides, the strength of which is controlled by the Fe-spin state. Reducing the Fe-magnetic
moment, weakens the Fe-As bonding, and in turn, increases As-As interactions, causing giant
reduction in the c-axis. For
CaFe2As2
system, this reduction is as large as 1.4 Ang. (~13%).
Since the large c-reduction has been recently observed only under high-pressure,
our results suggest that the iron
magnetic moment should be present in Fe-pnictides at all times at ambient pressure.
The giant coupling of the on-site Fe-magnetic moment with the As-As bonding
that we have discovered here may provide a mechanism for the superconductivity.
Read more:
T. Yildirim, Phys. Rev. Lett. 102, 037003 (2009) and
arXiv:0807.3936v2
(PRL's extended version)
Nature and Tunability of H2-Binding in MOFs
W. Zhou and T. Yildirim, J. Phys. Chem. C, 112 , 8132 (2008)

MOF compounds with exposed transition-metal (TM) sites exhibit impressive heats of adsorption of H2,
thus the TM-H2 interaction was believed to be of the "Kubas-type". Our calculation shows that the H2 binding
in Mn4Cl-MOF is not of the expected Kubas-type. Instead, the major contribution to the overall binding
comes from the classical Coulomb interaction, which is not screened due to the open-metal site.
We also show that the orientation of H2 has a surprisingly large effect on the binding potential,
reducing the classical binding energy by almost 30%. Read more...

We conducted a systematic study of the H2 adsorption on a series of isostructural
MOFs, M2(dhtp) (M=Mg, Mn, Co, Ni, Zn). The experimental Qst for H2 of these MOFs range from 8.5 to
12.9 KJ/mol, with increasing Qst in the following order: Zn, Mn, Mg, Co, and Ni.
The H2 binding energies derived from DFT calculations follow the same trend.
We also found a strong correlation between the metal ion radius, the M-H2 distance
and the H2 binding strength, which provides a viable, empirical method
to predict the relative H2 binding strength of different open metals.
Read more...
Metal-Ethylene Complexes for Hydrogen Storage In our recent studies
[Phys. Rev. B 76, 085434 (2007),
Phys. Rev. Lett. 97, 226102 (2006)],
we suggest that co-deposition of metals (i.e. Ti/Li) with small organic molecules such
as very cheap ethylene molecule (a ho-hum material that is the building block of the
most common plastic) into nanopores of low-density high surface materials could be
a very promising direction for discovering new materials with better storage properties.
We found to our surprise that the interaction of Ti with the C=C double bond of
ethylene molecule (i.e. C2H4 ) mimics what we found in C60.
Detailed first-principles calculations show that the complex resulting
from attaching a Ti atom to each ends of C2H4 (see figure) will reversibly
adsorb ten H2 molecules. The equivalent material gravimetric capacity of 14%,
if realized in practice, would readily exceed the 2015 DOE system goal.

Another advantage is that, unlike our previously predicted structures
involving fullerenes and nanotubes, the metal-ethylene complexes
have actually been synthesized and actively studied as catalytic
systems for several decades. Their potential for hydrogen storage
was first revealed by our theory and modeling work.

Some preliminary experimental results demonstrating 14 wt% H2 adsorption
at 300K on Ti-C2H4 complexes formed by laser ablation has been recently published in
Phys. Rev. Lett. 100, 105505 (2008).